The invention relates to the forming of extrusion dies, such as dies used for extruding aluminium.
It is well known that in an extrusion die which is of substantially constant axial bearing depth between the inlet and outlet, the resistance to the flow of material through the die varies in different parts of the die aperture in accordance with the shape of the aperture. For example, the resistance to flow between two opposed parallel walls will be generally inversely proportional to the spacing between the walls. In addition, the extruding pressure normally varies across the die aperture. These variations in the resistance to flow and extrusion pressure in different parts of the die aperture lead to different rates of flow in the material being extruded, leading in turn to severe internal stresses in the material. It would therefore be advantageous if an extrusion die could be so designed that the rate of flow of the material being extruded was substantially uniform over the whole area of the aperture.
It is known that the resistance to flow in any part of an extrusion die may be reduced by reducing the axial bearing depth of the aperture, or increased by increasing the bearing depth. It has accordingly been the practice to design extrusion dies so that the bearing depth is reduced in areas where the shape of the aperture is such that resistance to flow will be high, and/or where the extrusion pressure is low. However, there has hitherto been no readily available method of determining how precisely the bearing depth should be varied over the die aperture to achieve the required effect. Instead the design of dies has depended on the judgment and experience of the designer followed by trial and error testing and modification of the die. This is obviously time consuming and, at best, can achieve only an approximate result.
It has been proposed that the required correlation between the shape of a die aperture and its bearing depth might be determined by computer analysis and while it is probable that this could provide a solution, a great deal of work is likely to be involved in writing the necessary programs and, in any case, the use of expensive computer equipment is necessary. Furthermore, even when the required dimensions of the die aperture have been thus determined, the necessary shaping of the die in accordance with the computed requirements is likely to be difficult.
The present invention provides a simple method of determining, in an extrusion die, the bearing depth required in different parts of the die aperture to provide substantially uniform flow across the die aperture and also, in a preferred embodiment, provides a simple method of shaping the die to the required dimensions.